An ultrasound system has become an important and popular diagnostic tool since it has a wide range of applications. Specifically, due to its non-invasive and non-destructive nature, the ultrasound system has been extensively used in the medical profession. Modern high-performance ultrasound systems and techniques are commonly used to produce two-dimensional or three-dimensional ultrasound images of internal features of a target object (e.g., human organs).
The ultrasound system may provide ultrasound images of various modes including a brightness mode (B mode) image representing reflection coefficients of the ultrasound signals reflected from a target object of a living body with a 2D (two-dimensional) image, a Doppler mode (D mode) image representing speed of a moving target object with spectral Doppler by using a Doppler effect, a color Doppler mode (C mode) image representing speed of a moving target object with colors by using the Doppler effect, and an elastic mode (E mode) image representing mechanical characteristics of tissues object before and after applying a pressure thereto. Particularly, the ultrasound system may transmit and receive ultrasound signals to and from the living body to thereby form Doppler signals corresponding to a region of interest (ROI), which is set on a B mode image. The ultrasound system may further form a C mode image that represents the speed of the moving target object such as blood flow, heart, etc. with colors based on the Doppler signals.
The color Doppler image may be formed based on Doppler signals obtained by alternately transmitting and receiving ultrasound signals to and from a target object. The Doppler signals may include a low frequency signal (so-called clutter signals) due to the motion of a cardiac wall or valve of a heart. The clutter signals may have amplitude, which is over 100 times than that of the blood flow signals indicative of velocities of the blood flow. The clutter signals may be an obstacle for accurately detecting a velocity of the blood flow. Thus, it is required to remove the clutter signals from the Doppler signals for an accurate velocity detection of the blood flow.
The ultrasound system typically adopts a clutter filter that may be a high pass filter to remove the clutter signals from the Doppler signals. However, this presents a problem since the clutter filter cannot completely remove the clutter signals from the Doppler signals.